1. Field of the Invention
The present invention relates to a parison controller of blow molding apparatus and, more particularly, relates to a novel improvement thereof in which a core provided at an ejection orifice for ejecting the parison is moved by a motor, to achieve reduction in size of the apparatus as a whole and saving of resources and to obtain a high quality parison.
2. Description of the related art
Various types have conventionally been used as a parison controller in such a blow molding apparatus, and typical ones include a generally employed construction as shown in FIGS. 1 and 2, which will now be described.
Referring specifically to FIG. 1, what is denoted by numeral 1 is a cross head to which a molten resin 4 is supplied from an extruder 2, and formed on a die 3 provided at the lower portion of the cross head 1 is an ejection orifice 5 for performing the ejection of the molten resin 4.
The ejection orifice 5 is formed in the shape of a ring by a taper-like core 6 which is operably provided at the center of the die 3. The core 6 has a taper portion 6a and a linear portion 6b integrally formed thereon as shown in FIG. 2, and the thickness of a parison 12 to be extruded may be varied by displacing the core 6 in the direction of arrow "A" to change at will the distance between an outer surface 6aA of the taper portion 6a and an inner wall 5a of the ejection orifice 5.
The core 6 is connected to a rod 7 provided through a top portion 1a of the cross head 1, the rod 7 being connected to a piston (not shown) of a servo actuator 9 which is provided above the cross head 1 by means of support pillars 8.
A hydraulic unit 10 and a control unit 11 are connected to the servo actuator 9 and a memory (not shown) in the control unit 11 contains a program for program-controlling the servo actuator 9.
The servo actuator 9 is operated in accordance with the program in the control unit 11 to control up and down movement of the core 6, so as to continuously effect control such that the thickness of the parison 12 to be ejected from the ejection orifice 5 may be changed at will to a thickness which is required corresponding to the shape of a formed product.
Since the conventional parison controller in a blow molding apparatus is constructed as described, the following problems occur.
That is, since the displacement of the core is effected by hydraulic control, the parison may be contaminated for example when a leakage of oil has occurred, the contamination of the parison being a fatal defect in forming. Further, hydraulic control is inefficient and cannot control the position of the core with precision, whereby it is difficult to achieve a highly accurate control of the thickness of a parison. Furthermore, cost of a hydraulic unit is high and it requires a large space, and the size of the motor of the hydraulic unit becomes large where it is impossible to achieve saving of energy.